1. Field of the Invention
This invention relates to a voltage-to-current converter that is also named as transconductor with wide linear range, especially relates to a low-voltage source, tunable transconductor with wide linear range transfer curve.
2. Description of the Prior Art
One basic converter named as voltage-to-current converter is designed according to an ideal voltage-control-current-source that has an output current I.sub.O and an input voltage V.sub.I. The ratio I.sub.O /V.sub.I denoted as G.sub.m (transconductance) which has nothing to do with the load resistor and the source resistor in the equivalent circuit of the ideal voltage-control-current-source. The voltage-to-current converter is an amplifier, which can produce an output current proportional to the input voltage. The ratio I.sub.O /V.sub.I mentioned above is equal to the transconductance (G.sub.m) of the amplifier.
A transconductor is a type of voltage-to-current converter, which has the gain G.sub.m depending on the bias current and the size of the transistor. As shown in FIG. 1, the aspect ratios of the transistors Q.sub.1, Q.sub.2, Q.sub.3, and Q.sub.4 are 10.mu./2.mu., 10.mu./2.mu., 10.mu./5.mu., and 10.mu./5.mu. respectively, wherein the unit is micro-meter (10.sup.-6 meter:.mu.m). The input stage is composed of transistor Q.sub.1 and Q.sub.2, and the load stage is not shown in FIG. 1. The output current I.sub.O of the transconductor is equal to the subtraction (I.sub.p -I.sub.n), and is also equal to G.sub.m .times.(V.sub.ip -V.sub.in). The voltage V.sub.ip denotes the voltage at the gate ip.sub.3 of the transistor Q.sub.3, and the voltage V.sub.in denotes the voltage at the gate in.sub.4 of the transistor Q.sub.4. The transconductance G.sub.m depends on the current I.sub.bias and the size (aspect ratio) of every transistor. The current I.sub.bias is the current used to bias the differential pair. The gain of the transconductor mentioned above is the transconductance G.sub.m.
As well known in the prior art, the transfer curve (I.sub.p versus V.sub.ip or I.sub.n versus V.sub.in) has a linear region that the current is linearly proportional to the voltage. The slope of linear region is the transconductance (gain) of the transconductor, and the transconductance of the amplifier can be controlled by adjusting the input voltage of the differential pair. The gain (transconductance) of the transconductor mentioned above operates ether in a linear region or in a nonlinear region depending on the variation of the bias current (or input voltage) of the differential pair. The transconductor can be used only when the differential pair is operating in the linear region of the transfer curve. It is preferred that the transconductor has a wider linear region of the transfer curve.
The circuit diagrams of various types of the traditional transconductor are shown in FIG. 2A-FIG. 2B. The circuit diagram of the first type of the degeneration transconductor is shown in FIG. 2A, and the circuit diagram of the second type of the degeneration transconductor is shown in FIG. 2B. In FIG. 2A, a resistor R electrically couples the sources of the transistors Q.sub.5 and Q.sub.6. The resistor R mentioned above can increase the range of the linear region of the transfer curve of the transconductor. In other words, when the transconductor shown in FIG. 2A operates within an input voltage level larger than that of the transistor shown in FIG. 1, the gain of the transconductor shown in FIG. 2A can remain a fixed value. In addition, the aspect ratios of the transistors Q.sub.5, Q.sub.6, Q.sub.7, Q.sub.8 and Q.sub.9 shown in FIG. 2A are 30.mu./2.mu., 30.mu./2.mu., 30.mu./2.mu., 30.mu./2.mu. and 30.mu./2.mu. respectively.
The drains of the transistor Q.sub.5 and Q.sub.6 are electrically coupled to the load stage (not shown) of the amplifier. The circuit diagram of an alternative type of the transconductor shown in FIG. 2A is illustrated in FIG. 2B. The resistor R in FIG. 2A is replaced by a transistor Q.sub.10 in FIG. 2B, and the transistor Q.sub.10 is controlled by the voltage V.sub.ctl at the gate of the transistor Q.sub.10. The transistor Q.sub.10 in FIG. 2B acts as the resistor R in FIG. 2A, and is of the aspect ratio 2.mu./20.mu.. The value of the transconductance of the transconductor shown in FIG. 2B can be controlled by adjusting the voltage (V.sub.tcl) at the gate of the transistor Q.sub.10. So the transconductance of the transconductor in FIG. 2B can be changed, whereas the transconductance of the transconductor in FIG. 2A is fixed.